The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Sidewall image transfer (SIT) is commonly used to form structures having high density patterns at sublithographic critical dimensions (CD) and pitches. During a SIT process, a mandrel layer is formed on a substrate such as a semiconductor wafer. The mandrel layer is lithographically patterned and etched to form a plurality of mandrels. A relatively thin conformal spacer layer is deposited over the mandrels. The conformal spacer layer is anisotropically etched to remove portions on upper surfaces of the mandrels and areas between the mandrels. Portions of the spacer layer formed on sidewalls of the mandrels are not fully etched. The mandrels are then selectively etched, leaving the sidewall spacers on the substrate. Because the sidewalls are formed when depositing the relatively thin conformal spacer layer, the sidewalls have a smaller CD and pitch than can be achieved by lithography alone. If the pitch and the CD of the mandrel are properly matched to the spacer thickness, the pitch of the structure formed via SIT is half of the lithographic mandrel pitch.